Does nesting habitat predict hatch synchrony between brood parasitic brown‐headed cowbirdsMolothrus aterand two host species?

Abstract

Nestling brown-headed cowbirds Molothrus ater typically hatch earlier and grow faster than young of the many host species of this generalist obligate brood parasite. However, a cowbird chick also benefits from the presence of some host nest mates as the parasite is provisioned disproportionately more with increasing brood size. Since asynchronous hatching affects both cowbird and host nestlings' growth and survival, mechanisms that optimize the timing of egg-laying by female parasites should be prevalent. Several habitat features might facilitate optimal timing of parasitic egg-laying and we examined whether aspects of host nesting habitat predicted cowbird hatching synchrony. We tested whether synchronous nests were less concealed, closer to perches, and located in areas of higher host density than asynchronous nests using a broad-scale information theoretic approach. There was no support for these predictions regarding song sparrow (Melospiza melodia; n055) or yellow warbler (Dendroica petechia; n067) nests parasitized by brown-headed cowbirds at Mono Lake, USA. For example, the best statistical models for predicting hatching synchrony in yellow warbler nests included nesting-patch width and nest-substrate shrub species. However, these relationships were relatively weak: both synchronous and asynchronous nests were in patches with statistically indistinguishable widths and the two dominant shrub species at our site contained similar proportions of synchronous and asynchronous nests. We conclude that the variability of host nesting habitats does not contribute to a biologically consistent effect on hatching synchrony by this generalist brood parasite.

Effects of parents and Brown‐headed Cowbirds (Molothrus ater) on nest predation risk for a songbird

“…An indirect measure of host detectability (hatching synchrony), however, was also unrelated with nest microhabitat in this system (Tonra et al. ). Evidence for direct predation by cowbirds does not negate the possibility that parasitism may also indirectly elevate predation risk for parasitized nests by conventional predators.…”